A flowmeter has been used in sensing flow rates of fluids such as kerosene, water, and gas consumed by households or companies. As the flowmeter, a flowmeter using a heat type (especially an indirectly heated type) flow rate sensor, whose cost can easily be reduced, has been used.
For the indirectly heated type flow rate sensor, a sensor has been used including a sensor chip constituted by using a thin-film technique on a substrate to stack a thin-film heating material onto a thin-film temperature sensing material via an insulating layer so that heat can be conducted between the sensor chip and fluid in a pipe (including a fluid flow passageway connected to an external pipe and disposed in the flowmeter). When the heating material is energized, the temperature sensing material is heated to change electric characteristics of the temperature sensing material such as a value of electric resistance. This change of the electric resistance value (based on a temperature rise of the temperature sensing material) responds to a flow rate (flow velocity) of the fluid which flows in the pipe. This is because a part of a heating value of the heating material is transmitted into the fluid, an amount of heat diffused into the fluid changes in accordance with the flow rate (flow velocity) of the fluid, the amount of heat supplied to the temperature sensing material accordingly changes, and the electric resistance value of the temperature sensing material changes. The change of the electric resistance value of the temperature sensing material also differs with temperature of the fluid. Therefore, another method comprises: incorporating a temperature sensing element for compensating for the temperature into an electric circuit for measuring the change of the electric resistance value of the temperature sensing material so as to minimize the change of a measured flow rate value by the temperature of the fluid.
This indirectly heated type flow rate sensor using a thin-film element is described, for example, in JP(A)-11-118566. In this flow rate sensor, the electric circuit including a bridge circuit is used to obtain an electric output corresponding to the flow rate of the fluid.
In the above-described flowmeter, a fin plate for heat exchange between the sensor chip and the fluid is projected into the fluid flow passageway. In the vicinity of the flow rate sensor, an electric circuit portion including a circuit substrate for flow rate calculation, a display portion, a communication circuit connection portion, and the like are disposed. A function portion of the flowmeter including these portions is entirely contained in a housing.
In the related-art flowmeter, a path for fluid circulation in the housing is not simply linear, and is formed in a bent shape in many cases. This is because a filter for removing foreign particles in the fluid is disposed in the circulation path, or the circulation path is formed so as to alleviate an influence of rapid temperature change of the inflowing fluid and to relatively moderate the temperature change of the flow rate sensor. On the other hand, the temperature in the housing is easily influenced by outside air temperature or direct sunlight. Therefore, there are problems that nonuniformity of temperature distribution by a position in the housing including the circulated fluid becomes remarkable and that accuracy of flow rate sensing easily drops.